In a DNA molecule, the two strands are not parallel, but intertwined with each other. Each strand looks like a helix. The two strands form a “double helix” structure, which was first discovered byJames D. Watson and Francis Crick in 1953. In this structure, also known as the B form, the helix makes a turn every 3.4 nm, and the distance between two neighboring base pairs is 0.34 nm. Hence, there are about 10 pairs per turn. The intertwined strands make two grooves of different widths, referred to as the major groove and the minor groove, which may facilitate binding with specific proteins.


Figure 3-B-3. The normal right-handed “double helix” structure of DNA, also known as the B form.

In a solution with higher salt concentrations or with alcohol added, the DNA structure may change to an A form, which is still right-handed, but every 2.3 nm makes a turn and there are 11 base pairs per turn.

Another DNA structure is called the Z form, because its bases seem to zigzag. Z DNA is left-handed. One turn spans 4.6 nm, comprising 12 base pairs. The DNA molecule with alternating G-C sequences in alcohol or high salt solution tends to have such structure.


Figure 3-B-4. Comparison between B form and Z form.


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